Steam turbine pressure rate limiter

Abstract

A steam pressure rate limiter for a steam turbine system reduces the setting of a steam control valve when a rate of decrease in steam pressure is detected which exceeds a threshold value of rate of decrease. A floor on control of the steam control valve is enforced in order to maintain a predetermined amount of steam flow to the turbine thereby to provide for cooling of the turbine.

What is claimed is: 
     
       1. Apparatus for limiting a maximum rate of pressure decrease in a steam turbine system of the type having a boiler, a steam control valve and a turbine comprising: a digital integrator including a clock effective to produce a clock signal having a predetermined frequency, an up-down counter, and gating means, said gating means being effective to feed said predetermined frequency to said up-down counter in a first sense to produce one of counting up and counting down when said output of said integrator exceeds a signal related to said steam pressure and being further effective to feed said predetermined frequency to said up-down counter in a second sense to produce the other of counting up and counting down when said output of said integrator is less than said signal related to said steam pressure whereby a count is stored in said up-down counter; and   means for reducing a setting of said steam control valve in proportion to an amount by which said count in said up-down counter differs from said signal related to said steam pressure.   
     
     
       2. Apparatus for limiting a maximum rate of pressure decrease in a steam turbine system according to claim 1 wherein said predetermined clock frequency is effective to establish a maximum integration rate, said maximum integration rate being effective to establish said maximum rate of pressure decrease. 
     
     
       3. Apparatus for limiting a maximum rate of pressure decrease in a steam turbine system according to claim 2 wherein said gating means includes means for differencing an output of said integrator and a signal related to said steam pressure, an output of said means for differencing being substantially zero when said rate is less than a predetermined value and related to the amount by which said rate exceeds said predetermined rate when said rate exceeds said predetermined rate. 
     
     
       4. Apparatus for limiting a maximum rate of pressure decrease in a steam turbine system according to claim 3 wherein said means for reducing a setting includes means for preventing setting said steam control valve to a completely cutoff condition whereby at least a minimum flow of steam to said steam turbine is maintained. 
     
     
       5. Apparatus for limiting a maximum rate of pressure decrease in a steam turbine system according to claim 3 wherein said means for preventing includes means for generating a floor signal related to at least a desired minimum position of said steam control valve and means for limiting an output of said means for reducing in response to said floor signal. 
     
     
       6. Apparatus for limiting a maximum rate of pressure decrease in a steam turbine system according to claim 5 wherein said means for generating a floor signal is further responsive to a valve position command signal for producing said floor signal. 
     
     
       7. Apparatus for limiting a maximum rate of pressure decrease in a steam turbine system according to claim 1 wherein said means for reducing a setting includes means for differencing a valve position command signal and a signal from said digital integrator to produce a desired valve position. 
     
     
       8. A method for limiting a maximum rate of pressure decrease in a steam turbine system of the type having a boiler, a steam control valve and a turbine comprising: counting pulses of a clock at a predetermined frequency in an up-down counter in a first direction when said pressure exceeds a count in said up-down counter and in a second direction when said pressure is less than said count;   differencing said count and said pressure to produce a pressure rate signal, said predetermined frequency providing a threshold value of rate of steam pressure change which must exist to produce said pressure rate signal; and   reducing a setting of said steam control valve in response to said pressure rate signal.